The subject matter described herein relates generally to integrated circuits and, more particularly, to methods and apparatus for fabricating integrated circuits that facilitate electrical isolation of components on a semiconductor wafer, chip, or die, and facilitate use of the integrated circuits in high-temperature environments.
At least some known silicon carbide (SiC) integrated circuits include a wafer substrate that includes at least one semiconducting layer. The substrate is sometimes referred to as the body and may be fabricated from a p-type material, an n-type material, and/or a semi-insulating material. Further, the SiC integrated circuits may, or may not, have an epitaxial layer formed on top of the substrate.
Many of these known SiC integrated circuits include a plurality of electronic devices, for example, transistors, resistors, and diodes, and include a body terminal embedded in a portion of the substrate. Such body terminals share the same substrate and are therefore substantially electrically coupled. However, the substrate can only be maintained at a single voltage and the body terminals of the transistors and the substrate are maintained at the lowest voltage potential of the plurality of transistors to facilitate proper operation of the integrated circuit. Energizing the substrate to a particular voltage is often referred to as back-biasing. The source terminals of each transistor can be energized to voltages different from the substrate. Therefore, the source-to-body voltage differential, i.e., VSB, sometimes referred to as a reverse-bias voltage, is maintained at approximately 0 volts (V) or higher. Otherwise, if the body voltage exceeds a source voltage for a transistor, the body and source junction may operate as a diode and current paths will form between source terminals of different transistors,
When the source voltage exceeds the body voltage, an incremental increase in VSB facilitates an incremental increase in the VTH of the transistor, thereby necessitating an incremental increase in a gate-to-source voltage (VGS) to overcome the increased VTH. Furthermore, an incremental increase in VTH of the transistor facilitates an incremental decrease in a drain-to-source (or, source-to-drain) current. Therefore, body voltage has an effect on the operation of the affected transistor, and the body acts as a second gate. Such effect is referred to as the “body effect”.
In some known integrated circuits, in order to facilitate conditions such that VSB is a positive value, each transistor includes a hard-wired interconnection to each associated body terminal to attain the lowest voltage potential required for that particular set of transistors. These connections increase the interconnect complexity of the integrated circuit. The additional hard-wired interconnections increase the die area required for fabrication of the integrated circuit, decrease a yield per wafer, chip, or die, and increase a cost of integrated circuit fabrication.
Moreover, many known integrated circuits include other electronic devices, for example, resistive devices such as resistors that include resistive properties that are voltage and temperature dependent. Therefore, varying voltage conditions associated with the common substrate during dynamic operation of the integrated circuit induces variations in the resistance of the resistive devices, and thus detrimentally affects circuit performance. Furthermore, varying environmental conditions associated with the technical or industrial application of the integrated circuit may include significant temperature variations that will also vary the resistance. Anticipation of such varying circuit voltages and temperatures impose either more restrictive constraints on integrated circuit design and fabrication of the circuits, more restrictive constraints on industrial applications, or more complex and costly fabrication materials and techniques.
Furthermore, many known integrated circuits are limited to operating temperatures of approximately 175 degrees Celsius (° C.) (347 degrees Fahrenheit (° F.)), while many industrial applications include environments that exceed 175° C. Hardening integrated circuits to be more robust in such high-temperature environments significantly increases design and fabrication costs of such circuits.